Current Near-Field Communication (“NFC”) transceivers are either equipped with a poor or no channel combiner unit.
Instead, NFC transceivers select the strongest channel only or alternatively form a combined information source by simply adding the channel information together. This current approach is suboptimal in terms of detection probability, sensitivity bounds and SNR.
In a first approach, selecting the strongest channel may lead to false detection when the signal is weak or when one channel contains additive noise that is stronger than the received signal.
The above described approach can easily occur in a direct conversion receiver architecture, where the receiver (“Rx”) input is sampled with the carrier signal in order to convert it to the baseband signal. During the down conversion operation, when one channel samples close to the zero-crossing instant of the carrier, high residual phase-noise contribution called phase modulation to amplitude modulation (“PM-to-AM”) conversion may occur.
In the second approach, combining the information sources by adding the channel information together may be problematic when the channel phase delays and group delays are different which occurs by the coupling system physics visible by different upper and lower sideband levels.
In a third approach, using two independently running single sideband receivers where the final output of the two channels are synchronized and added together may be complex, expensive, and synchronization may fail when one channel provides a lower SNR.
FIG. 1 illustrates a current NFC frontend architecture 100.
The NFC frontend architecture 100 includes an antenna 101, a high frequency attenuator (“HF-ATT) 102, mixers 103 and 104, baseband filters (“BBF”) 105 and 106, baseband amplifiers (“BBA”) 107 and 108, analog-to-digital converters (“ADC”) 109 and 110 and a digital signal processor (“DSP”) 111.
The load modulation amplitude (“LMA”) of the NFC target, or passive card or ticket reaches the antenna 101 with an arbitrary phase and magnitude. After input range adjustments by the HF-ATT 102, the signal is down converted using a Nyquist sampling IQ down conversion method.
The resulting in-phase and quadrature samples are band-pass filtered using BBF 105, amplified using BBA 107 then passed through the ADC 109 and processed in the digital domain by the DSP 111.
The applied Nyquist sampling method has moderate complexity and strain is put on the phase noise performance of the reference clock.